PURPOSE: To prevent overrun of a vehicle and the like caused by increase in the number of revolutions by supplying required auxiliary air which is less than maximum auxiliary air at the downstream side of an intake throttle valve in the case a maximum driving signal is supplied to a driving means.

CONSTITUTION: In the case a maximum driving signal is inputted to driving means(coils) 10a, 10b, the opening area of at least one of an outlet hole 44 of a first bypass passage 1a or an inlet hole 45 of a second bypass passage 1b is varied by a valve body 5 driven by the driving means, 10a, 10b. The valve body 5 is controlled and auxiliary air is adjusted under an idling condition, and the number of revolutions is controlled to be the target revolutions.